Interview with Kerry Emanuel

Interviewer: I’d like to begin by asking you how you first
got interested in weather phenomenon. Can you think back to what made this
a lifelong interest for you?

KERRY: As is true of a lot of people in my field, I got
interested in it as a child. I grew up around New England. And New England
has a lot of weather. It's very variable. And I got interested in weather
phenomenon. I just wanted to try to understand how various things work.

Interviewer: Is there a particular time when you first began to look
at hurricanes as a phenomenon interest?

KERRY: Yes, actually. This is a classic example of how sometimes
you get interested in something by having to teach it. And, one of the reasons
that teaching and research go together so well is that, often, you think you
understand something. And it's not until you have to teach it that you understand
that you don't understand it.

And this happened to me in the case of hurricanes. So I was asked to teach
a course in Tropical Meteorology. I thought I understood how hurricanes work.
I knew what the conventional explanation was. So I started to teach it. And
I had one of those horrible experiences where you start talking about something,
and you realize it doesn't really make any sense. And, that leads you down
a path of trying to understand what does make sense. And that's what we call
research.

Interviewer: Have you ever looked at how hurricanes are usually taught
in school to kids? And how that description could be expanded?

KERRY: Well, yes. I think in almost all presentations I'm
aware of, whether they're on television, or in a documentary film, or in a
textbook, it's very descriptive. This is a sequence of events that we observe
happen that leads to a hurricane. And it's not really explanatory. There's
very little that says, "Well this is why it's happening. This is the
physics behind it." So, I see that as missing from a lot of the curricula
on hurricanes.

Interviewer: Let's go into that a little bit. From a physics standpoint,
what is a hurricane?

KERRY: Well, from a physics standpoint, a hurricane is
a heat engine. It's a massive, natural machine for converting heat energy
into mechanical energy, the mechanical energy being the energy of the wind.
And the heat energy is derived from the ocean, and specifically from the evaporation
of ocean water. Now, it's not always clear to people that when you evaporate
water, this actually corresponds to a transfer of energy. And yet we all have
that experience.
For example, when you get out of a swimming pool on a windy day, even though
it's hot, you might feel cold for a while because water is evaporating from
your skin, and it's taking heat out of your body. And when water evaporates
from the ocean, it takes heat out of the ocean. And that heat is actually
added to the atmosphere, and it's a form of heat we call "latent,"
because it isn't really apparent. It's expressed in the water vapor content
of the air.
Now later, when that water vapor condenses into little drops of liquid water
that we call "cloud," that heat is released, and warms the air.
And so it's the terrific amount of heat released in the inner core of a hurricane
where all this water vapor is condensing. That is the proximate thing that
drives this grand circulation.

Interviewer: Can you just talk a little bit about what heat is converted
to.

KERRY: Let me draw an analogy which is much more familiar
to people in everyday life, which is an automobile. An automobile is a machine
that converts, ultimately, chemical energy, the energy stored in gasoline,
first to heat energy when a little miniature explosion occurs inside the cylinder
of the engine. And then that heat energy is converted to mechanical energy
which is the motion of the car.

Now, when you step on the gas, the car is accelerating. It's this energy
that's being used to accelerate the car. When you're going down the highway
at 60 miles per hour, and you're no longer accelerating, you're stilling burning
up energy. It's all the energy you need to overcome the friction of the car
moving down the road, and the friction of the air moving past the car.

There's an analogy with a hurricane. A hurricane spins up, and it reaches
some maximum intensity. And at that point, it's no longer growing. But, there's
an enormous amount of friction, of the winds blowing across the surface of
the earth. And, it's the heat energy that the hurricane thrives on that, in
this case, is being first converted into the mechanical energy of the winds,
and then dissipated by friction.

And that dissipation, by the way, turns the energy back into heat. Friction,
you rub your hands together, they get hot. Friction causes heat. So it's a
cycle.

Interviewer: So when you're experiencing the fastest, heaviest winds
of a hurricane, there's friction there and it's heating up.

KERRY: That's right. Some of the water that's evaporated
from the ocean, and transferring heat into the air, some of that heat is being
used to push you. It's the energy that's being used to push you, or drag you
across the ground if you can't stand up in such high winds. We can talk about
the energy that's dissipated in a hurricane in sort of quantitative terms.
And the number that I like, because it just works out to be almost exact,
is that, for an average hurricane, not a particularly big one, not a particularly
powerful one, the amount of energy that's dissipated, frictionally, which
is equal to the amount of heat energy that's being pumped into this storm,
happens to be almost exactly equal to the average rate at which the United
States uses electrical power. So the whole generating capacity of the United
States is equivalent to one hurricane.

Interviewer: How is a hurricane like a heat engine?

KERRY: One of the principles that was established in the
very early days of thermodynamics, back in the middle of the 19th century,
mostly by the French, was that you could only take a fraction of heat energy
and turn it into mechanical energy. And, this fraction is called the "efficiency."
And, this theory was developed by a French thermodynamicist by the name of
Sadi Carnot. And, that's why a lot of heat engines, even today, are called
Carnot Engines.

And this fraction is proportional to the difference between the temperature
at which the heat is put into the system, and the temperature at which, ultimately,
it's taken out. So, let's start with a simple example. Actually, it's not
very simple at all, but a familiar one. A familiar example is a car. Now,
heat is being added inside the cylinder of the engine where the temperature
is very, very hot because of all the explosions of gas vapor and oxygen mixture
in there.

And the heat is ultimately taken out of the car by your radiator. Your radiator
may be operating at a temperature of whatever the temperature is outside.
And it's a lot less than the temperature inside the cylinder. So the greater
that temperature difference, the greater the fraction of the total heat energy
you can turn into mechanical energy. That's an upper bound. There can be all
kinds of other inefficiencies in the system.

Now, a hurricane is also a heat engine. And the temperature at which the
heat it put in is the temperature of the ocean, on the average about 86°
Fahrenheit. So, in the case of a hurricane, it's also an example of heat engine.
The heat is put into the engine at the temperature of the ocean, the surface
of the ocean, which is around 86° Fahrenheit, or maybe about 28° Celsius.
And, the air flows up the eye wall of the hurricane, a ring of very strong
thunderstorms that surrounds the eye. And then it goes out at the much higher
altitude of the very upper part of the atmosphere where the temperatures are
really, really cold. Some of the coldest temperatures you can find anywhere
in the atmosphere, ironically, are in the tropics, about 10 miles, or 16 kilometers
above the surface. And the temperatures are as low as -80°C, which is
well below -100° Fahrenheit.

So there's a big temperature difference acting across the hurricane. And
for that reason, the hurricane is a fairly efficient heat engine, as natural
heat engines go. You're converting about a third of the heat energy you're
getting from the ocean into wind energy.

Interviewer: What is the ultimate source of the energy that drives
this heat engine? How is it related to the greenhouse effect?

KERRY: The ultimate source of energy for everything, virtually,
that happens in the atmosphere is the sun. And in fact, it's the sun in combination
with the greenhouse effect. And we would not have hurricanes were it not for
the greenhouse effect, by the way. Here's how it works. The sunlight heats
the ocean. And if there were no atmosphere, the ocean would get rid of its
heat simply by emitting radiation -- it's called "infrared radiation,"
radiation you can't see -- back into space. And it would be a simple balance.
That's, for example, the balance that exists on the surface of the moon and
the soil that's facing the sunlight. And the moon has a certain temperature
which you can easily calculate, so that's it's emitting just as much radiation
as it's getting from the sun.

But, the earth has this nice blanket of gasses around it. And some of those
gasses are very important greenhouse gasses, water vapor being the most important,
and also carbon dioxide, methane. And those gasses absorb some of the heat
coming back up from the planet, and then re-radiate it back down to the surface.

And so, in the tropics, there is so much water vapor that the ocean finds
it really can't get rid of the heat it's receiving from the sun by radiating
it into space. So instead, the heat is carried away by currents of air. The
air rising from near the ocean surface is a little bit warmer, and a lot moister
than air coming down in downdrafts. So the air is carrying the heat away.

And for it to do that, the air in contact with the ocean has to be not saturated
with water vapor. Its relative humidity has to be less than 100%, so that
there's a potential for evaporation. And it's this potential for evaporation
that drives hurricanes. It's also why hurricanes tend to be most intense and
frequent in the late summer and early fall when the ocean temperature is at
its maximum and this disequilibrium between the ocean and the atmosphere is
also at a maximum.
So if it weren't for the greenhouse effect, we wouldn't have a problem with
hurricanes. One of the many things we're worried about, if we should have
any sort of global warming because the greenhouse effect is increasing, is
that hurricanes might be a little bit more intense in a warmer world.

Interviewer: How do temperatures vary in a hurricane, vertically,
from the ocean surface to the top?

KERRY: If you look at a photograph of a hurricane from space,
what you see, of course, is this beautiful white spiral mass of clouds. It's
a little bit hard to tell, but those clouds extend from very close to the
ocean surface up about ten miles high. And, the ocean surface is at a pretty
high temperature. If anybody's been to the tropics, they know that. It's in
the upper 80's Fahrenheit, or 25° to 30° Centigrade. But the tops
of the clouds in a hurricane represent the temperature that you find in the
upper part of the atmosphere. It's very, very cold. It's around -80° Centigrade,
or well below -100° Fahrenheit.

And, it's something not too many people know about, the tropical atmosphere.
It's true everywhere in the atmosphere. It gets colder as you go up. That's
why there's snow on top of Mt. Kilimanjaro, which is nearly at the Equator.
But if you go very far up, way above the tops of mountains, it's even colder.
And, it can be very, very cold. So, the hurricane is operating across a rather
spectacular gradient, from the ocean surface to the top of the storm.

Interviewer: Could you describe the process of hurricane formation?

KERRY: In fact, the generation of hurricanes, we call the
"genesis of hurricanes," remains one of the most enigmatic problems
in atmospheric science. We don't really understand how it works. But this
is what we see when we look at things like satellite pictures, or sequences
of pictures. Typically the first thing you see is a cluster of thunderstorms,
or what we call "convective showers." These are big cumuli nimbus
clouds that are raining, very common in the tropics. And, usually they're
scattered, and they're kind of disorganized.

And what you see is that they start to form clumps. And every once in a
while, the clumps will begin to show some signs of rotation. And then, as
time goes on, they get organized into kind of an anulus, a donut shaped ring
of convection. And as the winds intensify at the surface, often an eye will
develop, a clear region maybe 20 or 30 miles across in the center where the
clouds go away, and you're left with this much more organized, much more symmetrical
mass of clouds.
Now, we understand that once this process gets underway, the feedback that
allows the hurricane to grow is one between wind and evaporation. We just
discussed a few minutes ago that the main source of energy for a hurricane
is evaporation of water. But just as you feel colder when you get out of a
swimming pool on a windy day than a calm day, the rate at which heat goes
into the atmosphere is a function of the wind speed. The stronger the winds,
the more heat you have.

So you have this loop. The stronger the hurricane winds, the more evaporation.
The more evaporation, the more heat goes into the hurricane. The more heat
goes into the hurricane, the stronger it gets. And that would just go on forever.
But there's one other thing that's stopping it from doing that. It's friction.
And friction goes up even faster with the wind speed than the evaporation
does. And eventually, it catches up. And at that point, the hurricane stops
growing.
What is mysterious about this -- I think we understand that process pretty
well, by the way. -- is how the whole thing got started. And, we know enough
now to know that it doesn't start spontaneously. That is, a butterfly flapping
its wings somewhere in the western part of Africa is not capable of starting
a hurricane. A hurricane is more like a lawnmower, in the sense that your
lawnmower is sitting in the garage. It's got perfectly nice gas in it. And
everything is oiled. The spark plugs are good. But fortunately for you and
for me, except maybe in horror films, the lawnmower does not just start spontaneously.
You have to go over and the give the string a pretty good pull. And everybody
who's tried to do that knows that if your pull is too anemic, it doesn't start.

Hurricanes are like that, too. You have to have some other kind of storm
that has nothing to do with any of the physics we've been talking about, that
comes along and gives a sufficiently strong push to the atmosphere. And, we
understand why that's true. But what we don't understand is how it all manages
to happen. And it's rare, fortunately. It doesn't happen all that often.

Interviewer: Tell us a little bit about some of the effects of a
hurricane when it hits landfall. How would you know, if you were standing
on the beach, that it was a hurricane, and not just a squall or something?

KERRY: Well, a hurricane is probably the most powerful
storm on earth. The wind speeds are not quite as high as a tornado, but they're
much more extensive. A tornado covers a few hundred meters, whereas a hurricane
spans many, many tens and hundreds of miles. And when a hurricane approaches
the shore, there are all kinds of things that happen.
The first indication that something bad is happening out at sea are these
terrific waves start coming ashore, long before there's any wind. The waves
typically outrun the hurricane. And, back in the old days, before we had satellites
and aircraft, when we didn't really know what was going on out in the sea,
weather forecasters relied very heavily on this.
There's the famous story of the Galveston hurricane that's commemorated in
this wonderful book called Isaac’s Storm about Isaac Cline, who was
the meteorologist on duty in Galveston in 1900. And, although he had reports
from Cuba that there was a hurricane there, they had no way of really knowing
whether it was coming into the Gulf of Mexico. The first time he really got
suspicious is when these enormous swells began to roll in to the beaches of
Galveston. And he started to think, "Oh my goodness. Something is going
to happen." So that's the first thing that happens.

Then of course, the sky out to sea turns very dark, as the central mass
of clouds approaches the shore. The wind gradually increases. It's not usually
a very sudden increase of wind. You start to get rain squalls, the spiral
bands of the hurricanes. So the rain comes and goes, and the wind gradually
increases.

And then as the eye wall approaches the shore, the winds start to increase
very, very fast, and particularly if the hurricane is moving fast. And the
other thing that happens is, of course, the waves get bigger and bigger. But
the waves are superimposed on another phenomenon that we call the "storm
surge," which is a general elevation of the water, which is caused by
the net effect of the winds pushing all this water in to shore.

And there's another small contribution just from the fact that the pressure
in the center of the hurricane is very low. It actually pulls the ocean surface
up a little bit. So this big wall of water comes in. And, it can be 20, 30
feet high in extreme cases. And historically, it's this storm surge that has
killed the most people, and, in many storms, does the most damage.

The other phenomenon that many people are not familiar with is the enormous
amounts of spray that are lofted into the air in a hurricane. When the winds
start to flow more than about 80 or 90 miles per hour, so much spray is lofted
into the air that, eventually, it becomes hard to even talk about the surface
of the ocean. And I'm not speaking metaphorically. You just go from air which
is filled with bubbles to-- water which is filled with bubbles to air which
is filled with spray gradually. And there's no longer anything you can call
the surface of the ocean. So it becomes a real chaos in the core of a hurricane.

Interviewer: What about the eye of the hurricane?

KERRY: Well of course, there's a lot of lore about the
eye. And, visually, the eye is the most spectacular place to be, particularly
if you're in an airplane. What happens is, just as the winds reach their most
ferocious state, they will quite suddenly start to decline. And, in a very,
very intense storm like Hurricane Andrew, that decline can be very rapid.
Almost in a few minutes, the wind just drops off. The skies may clear. Although
usually, there's a lot of low clouds around. You can sort of see the sun now
and then peeking through the clouds, which is too bad. Because if it weren't
for the low clouds, you could see this wall of clouds that just went over
you, and it would be very spectacular looking. I know because I've seen it
from an airplane.

The winds drop off. And in the old days before people really even understood
hurricanes were rotary storms, a lot of people got killed because they thought
the storm was over. So they went out, and they tried to start picking up after
all the damage, only to be hit by the other side of the storm. Today, most
people understand that and that doesn't happen.
But there are all sorts of other crazy things that happen in the eye. Birds
at sea will sometimes get trapped in the eye. And, because it's so violent
in the eye wall, they really can't get out. And if the hurricane goes to very
high latitudes, it takes all these tropical birds with them. So that every
once in a while, some tropical species of bird will show up in Newfoundland
or someplace.

And if a ship happens to mistakenly get caught in the eye of a storm, which
doesn't happen very much anymore because of the good warnings they get, then
these birds -- exhausted birds -- will land on masts on the ship. There are
photographs of these tankers in the eyes of hurricanes completely covered
with birds that were trying to get a rest.

Interviewer: How would can you predict the intensity of a hurricane
as it evolves?

KERRY: Well, when we first started to look at this whole
business of treating the hurricanes as systems, one of the things that came
immediately out of that was that we realized that for a given atmospheric
environment, in a given ocean temperature, there was a maximum wind speed
that you could achieve in a hurricane, which we can easily calculate if we
only know the sort of average properties of the tropical atmosphere in the
ocean. It's a speed limit.

And, in some sense, it works very well. That is, if you look at the actual
observed wind speeds in lots and lots of different hurricanes, and you compare
those wind speeds to this limit, indeed, no storm ever exceeds this limit.
And only a very small percentage of them actually come right up to the limit.
Most of them fall far short.

One of the interesting things that happened to us when we did this is we
started to build computer models of hurricanes. And, in fact, I have such
a model that I distribute freely to teachers that can be run on an ordinary
personal computer. [See: http://wind.mit.edu/~emanuel/home.html ] And, the
computer models behaved, in some ways, very differently from real hurricanes
in that they always spun up to their speed limit, unless something really
terrible happened to them, like they went onshore.

We didn't understand that at first. The ideal computer hurricanes always
reach their speed limit. But the real ones seldom made it. Even if you look
at the real hurricanes that stay over warm tropical water, and don't make
landfall, they don't usually reach the limit either. And, we started to ask
why. One of the first things that occurred to us is that in the idealized
models, we hold the ocean temperature fixed. We don't let it change. It's
just whatever it is.

But a real hurricane profoundly changes the temperature of the sea water,
not because it's taking heat out of the ocean. It is doing that and that does
cool the water. But it's maybe a tenth of a degree or so. It's not really
noticeable because it's such a huge heat reservoir. What the real hurricanes
do is they churn up the ocean. And you don't have to go very far down in the
tropical ocean before you find very cold water. It's only hot right within
the first hundred feet or so of the surface.

The hurricanes come along, and they mix this cold water up to the surface.
You can look at a satellite picture and see these really cold wakes that are
left behind by hurricanes. And so the hurricane is cooling off the ocean temperature.
And it would only have to cool it off about 2?° Celsius right under the
eye wall to kill the hurricane. So even a 1° drop is important.

We’ve learned how to include that effect in the computer models, including
the simple one that anybody can run. We have a way of dealing with that now
which, indeed, predicts that most hurricanes don't really come up to the speed
limit.

Interviewer: Do hurricanes ever get close to their theoretical maximum
speed limit?

KERRY: It's only in very exceptional circumstances they
do. One famous case is Hurricane Camille, which was one of the most violent
hurricanes every to strike the U.S. in history.

In 1969, it came roaring up the eastern side of the Gulf. And it smashed
into Mississippi with winds of something like 200 miles per hour. This was
a bit strange because in the Gulf of Mexico, the layer of warm water, most
of the time, is very, very thin. It's 20 or 30 meters deep. And normally,
a hurricane would just never get that intense.

But, there is a phenomenon in the Gulf of Mexico called a "loop current,"
which is a narrow current of very hot water that runs very deep. It’s
called a loop current because it comes up through the Yucatan passage right
up toward New Orleans. And then it just does a U-turn, comes down just off
the west coast of Florida, goes through the Straits of Florida, and becomes
the Gulf Stream. So it's a predecessor of the Gulf Stream.

And we figured out that what happened with Camille is that by sheer accident,
Camille went right up the center of this loop current where the warm water
runs very, very deep. And so there wasn't any cold water to churn up. It didn't
have this negative influence. And it just really went to town. But there are
other things as well.

One of the most mysterious things is the interaction of hurricanes with
other weather systems, particularly if the atmosphere has what we call, "wind
sheer," where the background winds, the everyday normal wind that you
have in the atmosphere, is changing with height, either direction and/or speed.
When hurricanes move into environments in which this background wind is changing
with height, they often weaken. And that's observed so often that there can
be little doubt of it. But we don't really understand the mechanism very well.
There's all kinds of speculation.

All of this has led to the problem that we're really not very good at forecasting
hurricane intensity change. We can forecast where it will go pretty well compared
to a lot of other things we do. But the intensity change is just not very
good at all. So there's a long way to go in understanding how storms interact
both with their atmospheric environment and with the ocean.

Interviewer: Can you describe some of the current cutting edge research
that you're doing now, and why it's important to people?

KERRY: We're working on several facets of hurricane behavior
that range from the microscopic all the way up to global. Starting in the
microscopic end, if you want to predict hurricane wind speeds, you have to
know how the transfer of heat through the ocean surface, the evaporation,
depends on wind speed. And we know how it does at low wind speeds, but we
don't really understand, nor do we have measurements of, what happens at high
wind speed.

The other thing you have to be able to do is to predict the drag that ocean
exerts on the air. And again, we know how to predict that at low wind speed.
But, at high wind speeds, we would have these gigantic waves. We don't know
how to do that. Everything we know so far teaches us that the major piece
of physics that has to be understood to do this correctly at hurricane wind
speeds is sea spray.

Sea spray, we think, is fundamental to a hurricane. When a spray droplet
goes up and partially evaporates, it gets very cold. It comes back down to
the ocean. It turns out that process transfers an enormous amount of heat
to the air. And so sea spray is a very efficient heat transfer mechanism.

But when a spray drop goes up into the air, it has to accelerate to the
wind speed. And in doing so, it exerts a drag on the air. So it's also a major
sink of wind energy for a hurricane. And so we have built a laboratory apparatus
in which we can simulate these very fantastic conditions, very high wind speeds,
air blowing across water. The air is filled with spray. We're trying to carefully
measure the friction and the heat transfer of the spray.

Now to jump to the completely opposite end of the spectrum, people have
worried for some years about whether climate change affects hurricane activity.
Were there more or fewer hurricanes during that last Ice Age, for example.
Will there be more or fewer hurricanes if we have global warming? But very
few people have asked the question, "Do hurricanes play some central
role in the climate?" We tend to think of them as freak storms that don't
really affect the climate in an important way.

Text books, by the way, are full of bad information about this; hurricanes
transfer lots of water from the tropics to the Pole. It's just not true. They're
minor players in the water budget of the earth. But, there's something rather
unexpected that's turned up from research - that hurricanes may have a profound
effect on the global circulation of the ocean.
The ocean works in a very different way from the atmosphere. But, here's what
we know. Of all the heat that gets transported by both the ocean and the atmosphere
from the tropics toward the Poles, on average about a third of it is transported
by the ocean, and about two-thirds by the atmosphere. And so the climate would
be very different if the ocean weren't transporting all this heat.

There is a lot of argument about how this works. One of the theories that's
being kicked around in very recent days is that, oddly enough, it's hurricanes
that allow this to happen. And the reasons are very subtle. But it was actually
proved about a hundred years ago that the only way you could make the ocean
transport a lot of heat from the tropics to the Pole is if you could turbulently
mix hot water in the tropics down into the deep tropical ocean.

Oceanographers have argued for years about what's doing this mixing. We've
done some calculations that suggest that global tropical cyclone activity
(tropical cyclone is a generic name for a hurricane) is what's doing this
mixing. Now if this is true, it means that we have to completely rethink our
understanding of how the climate works in general. Because in the big huge
computer models that are used to simulate climate, this mixing is just specified.
It's constant. It doesn't change with time or climate. It's specified out
of ignorance. We don't know what else to do.

If it's hurricanes that are driving this mixing, we have a different problem
all together. We have a different system dynamic because the hurricanes themselves
are functions of the climate. And so, we have built some simple models in
which we allow the climate to control the hurricanes, and the hurricanes to
control the heat flux in the ocean. And we get some very interesting behavior
that may explain things, for example, like the climate 50 million years ago
when the tropics were about the same temperature they are today but the poles
were much warmer. Instead of being around 0° Centigrade, they were something
closer to 15 or even 20°.

This is when there were crocodiles wandering around Greenland, and alligators
in London, and places like that. And this is a great mystery because the distribution
of sunlight over the earth was not appreciably different in those days. But
we think it was a very stormy climate with a lot of hurricanes, which was
driving a fantastic polar heat flux in the ocean that was responsible both
for keeping the tropics relatively cool, and for keeping the high latitudes
warm.

This is a new branch of research that now ties hurricanes into the whole
climate system in a way that might prove to be very interesting.

Interviewer: As you've worked on this, have you had your view of
the earth's circulation system become more of a bird’s eye view rather
than a microscopic view?

KERRY: I think it's worked in both directions. For me,
since I've been in this field, which is well over 20 years, I came from a
fairly narrow perspective of trying to understand a few types of systems we
see in the atmosphere. But, that understanding has led -- or the attempt to
achieve such an understanding has led -- both downscale and upscale. On the
one hand, we're forced to study what we call, "microscopic processes,"
like how does a drop of spray form and break off, and what laws govern that,
down to this whole global thing of how is everything interconnected in a way
that controls the climate?

And understanding climate is a really fascinating issue. It's a young science.
But in the last decade or two, all of these ice cores and deep sea sediments
have provided this marvelous record of climate that shows a very, very unusual
and dynamic system that we only are beginning to understand. So for anybody
starting out in science, this is a fantastic place to go and make discoveries.

Interviewer: Is there anything that you wanted to add that we didn't
ask you?

KERRY: There's one other interesting branch of research
that may help us understand the relationship between hurricanes and climate.
It's what's called paleotempestology. That's an effort to reconstruct a very
long-term record of storminess from the geological record. There are several
techniques, and one which I'm familiar with involves going to lakes [or lagoons
that are near the shore] but that are separated from the ocean by a sandbar.
There are lots of those along the U.S. Gulf Coast and the East Coast.

The nature of these lakes is such that when a hurricane does come, and there's
a direct hit, the storm surge washes over the sandbar into the lake, carrying
with it a lot of sand. And so a layer of sand is deposited at the bottom of
the lake. Now, the rest of the time, when there's no storm, it's just mud
that's building up, organic deposition, plants, decaying plants and things.

If you go into the lake with a boat and you drill down into the lake bottom,
and you take a core up and you look at that core, what you see is a lot of
black mud with some intervening layers of sand. Those layers of sand represent
hurricane strikes. You can use a technique that has to do with the radioactive
decay of certain isotopes and this organic matter to date it, so you can find
out when those sand layers were put down.

People have gone back in time now about 4000 years with this technique.
There are certain places along the Gulf Coast where we now have something
like a 4000 year record of hurricane activity, which is fantastic. One of
the things we see consistently is that the period between about 1000 years
ago and 3000 years ago was very active compared to today. There were a lot
more hurricanes than there have been in the last 1000 years.

During the period before about 3000 years ago there were very few storms.
So we had very few, then we had lots. And then in the last 1000 years, we
had few in that part of the world. If that same kind of thing can be done
all over the place, we could build up quite a long-term record of hurricane
activity. And that would be a step toward understanding what controls the
level of hurricane activity.